Fastening screws of the foregoing kind are manufactured in a variety of forms, and it is generally the case that different forms are adopted for penetration into metal and timber respectively. Fastening screws intended to drill through sheet metal have a drilling tip which has substantially the same characteristics in terms of shape and function as the end of a conventional drill bit for drilling metal (see the attached FIGS. 1 and 2). The end portion of the fastening screw at which the drilling tip is formed is not threaded and is very much like a conventional drill bit except that the flutes are straight rather than helical. The length of the non-threaded end portion varies, but is generally significantly greater than the diameter of that portion.
A fastening screw intended for penetration into timber has an entirely different drilling tip, and a typical example is shown by the attached FIGS. 3 and 4. The major features of the wood screw is that it has a tapered end portion which terminates in a pointed terminal end, and the thread extends along that end portion substantially up to the terminal end or tip. Also, a single flute is provided in the drilling tip rather than two as in the case of a fastening screw intended to drill through sheet metal. Continuation of the thread through the tapered end portion is an important feature because it enables the thread to bite into the timber early in the penetration process and thereby pull the screw into the timber. That reaction between the thread and the timber assists the drilling operation by requiring minimum endwise pressure on the fastener.
A problem arises when a fastening screw designed to drill through metal is required to penetrate into timber, for example when the screw is being used to fasten a metal sheet to a timber support. Under those circumstances the screw typically penetrates both the metal sheet and the timber without difficulty until the threaded portion of the fastener shank reaches the timber surface. The thread does not bite into the timber as with the fastener of FIGS. 3 and 4, but tends to ride over the timber surface. Substantial endwise force is then required to drive the fastener further into the timber and achieve self-tapping without significant stripping of the internal thread so formed.
Because of the foregoing problem it is not uncommon for tradesmen to use fastening screws of the kind shown by FIGS. 3 and 4 for securing metal sheet to timber. Substantial endwise force is required to initiate penetration through the metal, but less effort is required for penetration and tapping into the timber for the reason previously stated. The drilling tip of such fasteners is not suited for cutting into metal and tends to leave a burr as the fastener penetrates through the metal sheet. Such burrs are unsightly and are dangerous because of their jagged nature. They also promote corrosion of the metal sheet and can tend to cause the roof to leak, because the sealing washer is prevented by such burrs from sitting firmly against the roof surface.